This invention relates to a signal decoder and more particularly to a stereo signal decoder for use in a receiver which is capable of receiving compatible AM stereo radio frequency (RF) broadcast signals, wherein an RF carrier has amplitude modulation (AM) representing stereo sum (L+R) information and phase modulation (PM) representing stereo difference (L-R) information.
In my prior U.S. Pat. No. 4,018,994, an AM stereo receiver is disclosed for obtaining L and R information from an independent sideband (ISB) AM stereo broadcast signal of the above-described type. In such an ISB signal, left (L) stereo information is transmitted primarily in the lower sidebands of the composite modulated RF signal and right (R) stereo information is transmitted primarily in the upper sidebands of the composite RF signal. This results from a 90.degree. phase relationship that is introduced between the L+R and L-R modulating signals prior to their being used to amplitude and phase modulate, respectively, the RF carrier at the transmitter. In one type of ISB receiver, a corresponding 90.degree. phase difference is introduced between the demodulated L+R and L-R signals before they are matrixed to produce L and R output signals. The disclosure of U.S. Pat. No. 4,018,994 is incorporated herein by reference.
The receiver shown in my prior U.S. Pat. No. 4,018,994 is shown as being constructed from a plurality of separate electronic circuit components and achieves low distortion decoding of a received AM stereo signal by using a distortion cancelling technique in the stereo decoder. In accordance with one aspect of that technique, a received composite intermediate frequency (IF) ISB signal is inversely amplitude modulated as a function of the demodulated (L+R) signal. The resulting altered IF signal is applied to a synchronous quadrature detector, together with an IF reference signal that is developed by a PLL arrangement, where the phase modulation is demodulated to develop a distortion corrected (L-R) signal. The L+R and L-R signals are applied to a pair of 90.degree. phase difference networks and then matrixed to develop left (L) and right (R) stereo audio output signals.
In constructing AM stereo receivers of the aforementioned type it would be desirable to implement the stereo decoder using a single custom-built IC which incorporates all or many of the necessary circuit functions. Although this would substantially reduce space, power, cooling and weight requirements, the capital investment and time required to design and produce such a custom IC is substantial. Alternatively, therefore, it would be desirable to be able to use an existing, low-cost, readily available IC as the basis for AM stereo decoder configurations which would require far fewer discrete circuit components.
It would also be desirable to be able to implement such a simplified decoder using a PLL arrangement which will not introduce an undesirable tuning characteristic in continuously tunable stereo receivers. Generally, this requires some form of muting in the L-R signal path of the decoder during initial tuning of the receiver to a stereo station.
Accordingly, it is an object of the present invention to provide a simplified AM stereo signal decoder which makes novel use of an existing, low-cost IC to perform functions different from those for which it is intended.
It is another object of the present invention to provide an AM stereo signal decoder which incorporates a novel two-mode PLL configuration that has a wide pull-in range until the PLL is locked to the IF signal carrier component, and thereafter has a narrower hold-in range while the PLL remains locked to the received carrier. This PLL configuration also provides enabling of the stereo difference signal output when the decoder is in a condition for properly decoding stereo information.
It is still another object of the present invention to provide an AM stereo signal decoder which incorporates a novel "stereo bloom" feature whereby upon initially being tuned to a stereo broadcast the receiver operates in a monophonic mode and thereafter, following a selected perceptible delay, changes to a stereo mode.